This research studies environmental control of aggressive behavior. Attitudes concerning aggression (behavior that displaces another by injury, or by threat) range from belief that like movement and locomotion, it is simply "available"; to belief that it is structured and motivated by a biogenic drive analogous to hunger, to pain, or to anger. The research asks which of several hypotheses embodying that range of beliefs, best fits certain communal living conditions and circumstances minimizing and regulating fighting behavior. Because fighting behavior in humans is difficult to study under controlled conditions an animal subject was chosen; one selected, both in nature and artificially, for fighting ability; and yet one known to modify aspects of its aggressive behavior in simple learning situations. The Siamese fighting fish, Betta splendens, is one most suited to this purpose. Bettas, are highly aggressive, normally live alone in nature, and yet can live together in groups with little or no fighting. We successively introduce new individuals into a large communal aquarium at intervals long enough to permit establishment of "prior residency" amongst those already in residence. Systematic variation of prior exposure conditions of newly introduced fish is superimposed upon that basic procedure. This procedure not only minimizes injurious fighting, but permits variation of intraaquarium conditions, as well as the past experience of newcomers to a residency group. The objectives of the research are limited relative to the potential usefulness of the method. They are: 1) to describe the spatial and behavioral arrangements by which the residents go about the business of living together; 2) to describe and evaluate conditions that modify the interactions of newcomers with residents; and 3) to analyze the course of behavior during intial encounters between two fish strange to each other.